Type GUID = VARCHAR(255) on platforms that don't have a native GUID support

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I'm using MySQL with entities that have GUID ids. Therefore I'm using @ORM\Column(type="guid") for the ORM mapping. As MySQL does not have a native GUID data type, it gets mapped to type="string" with a default length of 255 -> VARCHAR(255). I don't really understand why we don't limit the length to 36, which is the fixed length for GUIDs. You could even think about using CHAR(36) for MySQL.